The present invention relates to assembly components such as bolts, rivets, nuts or washers, ties or tensioners notably with a cable, and other similar components which are subjected, when they are used to connect two parts, to a tightening tension. An assembly component of the type to which the invention relates may have a symmetry axis and two opposing faces. The invention also concerns assembly methods and machines, the latter term including the machines for checking the tension in the connections as well as manual tools.
The methods used up till now for measuring the tightening tension of two parts connected by an assembly component use either the measurement of the torque or the measurement of the tightening angle, or the combined measurement of the torque and tightening angle, if applicable together with a stoppage of the tightening at the elastic limit. None of these techniques directly measures the compression force or tightening tension, preferring the secondary parameters from which it is hoped to derive this tightening tension. However, experience shows that the law of correspondence between these parameters and the tightening tension is not reproducible, so that the measurements are vitiated.
In EP-A-381791 and U.S. Pat. No. 4,294,122, an ultrasonic measurement method is proposed for a bolt with a shank surmounted by a head. A piezoelectric sensor attached to the head is able to receive ultrasonic waves reflected by the free end of the shank, which for this purpose must be flat and perpendicular to the symmetry axis. The delay in the wave corresponds to the elongation of the shank. This complicated method is uncertain.
In GB-B-1315702, a method of measurement by positioning an electromagnetic apparatus on the bolt head is described. This method makes it possible to measure the bending deformation of the head. It does not make it possible to measure a radial deformation (perpendicular to the axis) under compression or traction since the apparatus is able to slide in this direction. As the sensor is not attached to the bolt head, during the rotation necessary for the tightening, signal instabilities will appear and thereby prevent feedback control. The smallest amount of dust interposed between the sensor and bolt head modifies the measurement conditions and introduces an error into the result. The method does not work on non-magnetic materials. Moreover, this equipment is bulky and expensive. An apparatus of this type can scarcely be allocated to a single assembly component and be sold with it.